The present invention relates to a technical field of correcting a tracking-error signal, and particularly to removal of an offset included in a tracking-error signal used for control, referred to as tracking servo control, executed in optically reproducing information recorded on a storage medium.
In general, for optically reading information recorded on a storage medium such as an optical disk using an optical beam such as a laser beam, two types of control referred to as a tracking servo control and a focus servo control are indispensable. The tracking servo control is to make it agree to each other both the focus position of an optical beam and the position of a track composed of information to be reproduced and formed on the storage medium, while the focus serve control is to make it agree to each other both the focus position and the position of an information-recorded area on the storage medium.
Practically, in the tracking serve control, reflected light formed by reflecting an irradiated optical beam to a storage medium is used to produce a tracking-error signal indicative of degrees in shift between the focus position of the irradiated optical beam and the position of an information track, and an objective lens for collecting optical beams is driven to bring the tracking-error signal into zero.
A theoretical truth is that the center in levels of the tracking-error signal represents the zero level when an irradiated position (i.e., converging position) of an optical beam is at the center axis of an information track, but the center level of the tracking-error signal may be shifted from the zero level. That is, an offset may be caused and included in the tracking-error signal on account of variability in characteristics of components of an optical system or electrical circuitry in an information reproducing system, or others.
A servo circuit responsible for tracking servo control using such tracking-error signal is designed on a precondition that such offset is not caused. Thus, a tracking-error signal including the offset results in that, when it is supplied to the servo circuit, the servo control causes the converging position of the optical beam to shift from the center axis of an information track by an amount which corresponds to the offset included.
A recent development in the digital signal processing technique leads to a tendency to digitize servo circuits for the tracking servo control. In such digitized servo circuits, the tracking-error signal produced is digitized, then supplied to servo circuits.
However, if the tracking-error signal which includes an offset is digitized, there occurs a problem that a linearity in an offset side in the digital conversion characteristics is not guaranteed.
For example, in cases that an AD converter has a performance of resolution of 8 bits (the number of quantization), a tracking-error signal provided in the form of analog quantities undergoes the analog-to-digital conversion with a condition that the center level of the signal is assigned to xe2x80x9c80HEX (in the hexadecimal notation)xe2x80x9d in a dynamic range of the converter, a plus-side maximum amplitude is assigned to xe2x80x9c00HEX,xe2x80x9d and a minus-side amplitude is assigned to xe2x80x9cFFHEX,xe2x80x9d respectively. If the tracking-error signal contains an offset, a maximum amplitude in the offset side will exceed the designed dynamic range of the A/D converter, the excessive signal part is fixed to a certain value, without being digitized.
To avoid this, a conventional technique, referred to a tracking balance circuit, is provided, wherein an offset component is extracted from an analog tracking-error signal before being supplied to a digital servo circuit, and the offset component is polarity-inverted and added to the tracking-error signal before being supplied to the servo circuit. Accordingly, the offset can be cancelled before digitization.
However, according to the conventional analog-type tracking balance circuit, there is a problem that it is difficult to perform an accurate tracking servo control, due to an offset remaining in the tracking-error signal digitized afterward, because the offset is not completely removed. Thus, with a condition that a slight amount of the offset which does not affect a linearity in digitization still remains in the analog tracking-error signal, the digitization also results in a digitized tracking-error signal including a xe2x80x9cremaining offset.xe2x80x9d
On one hand, there is another problem concerning the A/D converter itself. Variations in accuracy and other factors of such components as ladder-connected resistors arranged within the A/D converter causes a reference level of each A/D converter to deviate from its designed value. Hence, even when an offset is removed from the tracking-error signal, the center level of the tracking-error signal may still differ from the reference level of the A/D converter. This leaves the xe2x80x9cremaining offsetxe2x80x9d in the tracking-error signal, causing the same problem as discussed above.
Accordingly, it is an object of the present invention to solve the above problem, that is, to provide a technique for correcting a tracking-error signal, which is able to sufficiently remove an offset from the tracking-error signal in order to perform tracking servo control accurately, providing an accurate reproduction of information recorded on a storage medium.
In order to realize the above object, as one aspect of the present invention, there is provided an apparatus for correcting an offset included in an oscillating tracking-error signal indicative of a difference between a position of an information track formed on a storage medium with information to be reproduced and a position of an optical beam irradiated onto the storage medium to reproduce the information. In the apparatus, the tracking-error signal in an analog form is produced by producing means based on a reflected beam of the optical beam from the storage medium, a first offset-correcting signal is added to the analog tracking-error signal by first adding means such that the offset is removed therefrom, thereby producing an added tracking-error signal in the analog form, the added tracking-error signal is digitized by digitized means to produce a digital tracking error signal, and a second offset-correcting signal is added to the digital tracking-error signal by the second adding means such that the offset is removed therefrom, thereby producing an added digital tracking-error signal. Concurrently, in the apparatus, the offset corresponding to a difference between a center level of the tracking-error signal and a preset reference level is detected by the detecting means, the detected offset is compared to a threshold preset on a maximum amplitude of the tracking-error signal by the comparing means, and, by controlling means, the first offset-correcting signal is provided for the first adding means when the comparing means shows that the detected offset is not less than the threshold and the second offset-correcting signal is provided for the second adding means when the comparing means shows that the detected offset is smaller than the threshold.
Accordingly, the correction called coarse adjustment is first carried out by removing an amount corresponding to the first offset-correcting signal from the tracking-error signal before it is digitized, of which offset is still larger than the threshold. Then, the correction called fine adjustment is carried out by removing another amount corresponding to the second offset-correcting signal from the tracking-error signal after it has been digitized, of which offset has already been lowered to an amount not more than the threshold. This two-stage correction permits the offset included in the tracking-error signal for digital processing to be removed in a steady, stable and accurate fashion.
As another aspect of the present invention, there is provided a method of correcting an oscillating tracking-error signal used for positionally controlling an optical beam irradiated onto a storage medium to reproduce information recorded thereon, the tracking-error signal being produced in an analog form, and then converted into a digital form for use of control of the optical beam. The method comprises the steps of setting a first adjustment amount to correct an offset included in the analog-form tracking-error signal as well as a second adjustment amount to correct a further offset included in the digital-form tracking-error signal in the digital form, first correcting the analog-form tracking-error signal with the first adjustment amount, and second correcting the digital-form tracking-error signal with the second adjustment amount.
By this method, the foregoing two-stage correction is also provided, permitting the offset included in the tracking-error signal to be removed in a steady, stable and accurate fashion.
Still, as another aspect of the present invention, a system for reproducing information recorded on a storage medium is provided. The information reproducing system comprises an apparatus for correcting an offset included in the tracking-error signal, which is configured like the foregoing one, mean for tracking-controlling the irradiated position of the optical beam on the basis of the added digital tracking-error signal, and means for reproducing the recorded information on the basis of the reflected beam.
Therefore, the offset included in the tracking-error signal is steadily, finely removed, leading to an accurate reproduction of information recorded on the medium.